Abstract

Visual observations using camera systems on the tethered ROV Isis deployed during the 2008 JC024 cruise to the Mid Atlantic Ridge at 45°N showed numerous monogenetic volcanoes that are essentially piles of lava pillows. The pillows are usually ~1m diameter and >2m long and form mounds with average dimensions around 300m diameter, ~150m altitude, and 0.005km3 volume. Small protrusions, 10-50cm long, which are numerous on pillows appear to be the youngest regions, were sampled using the pincers on the hydraulic arms of Isis, and returned to the surface. On the surface, any glass crust on the pillow protrusions was chiselled off using clean tools and double bagged in polythene. In the laboratory a portion of the glass was crushed in a jeweller’s roller mill and sieved using stainless steel sieves to obtain a sufficient amount of the fraction 0.125-0.250mm for hand picking, using a binocular microscope with the glass submerged in a mix of water and iso-propyl alcohol. The samples were subsequently leached using the procedure of Standish & Sims. Samples were spiked with a mixed 229Th-236U spike and the U, Th and Ra fractions were separated and purified using standard chemistry methods. U and Th isotope ratios were determined using a Nu Instruments MC-ICPMS and Ra isotope ratios were determined using a MAT-262-RPQII TIMS instrument. The U-series data were evaluated using a MathCad program based on published equations. The data can be successfully modelled by assuming the ‘accepted’ mantle upwelling rate for the region of 11mm.y-1. The U-Th characteristics are mostly derived during ‘porous flow’ magma upwelling in the garnet stability zone, ranging to a depth of 60km with incipient melting starting at 70km. Above 60km depth the melt fraction will be >3% and the mantle mineralogy devoid of phases that fractionate U-Th significantly. Moreover, at melt fractions >3%, channel flow will be dominant and magma will transit to eruption on time-scales that are short enough to retain the U-Th characteristics from the garnet zone. The rheology of the deep mantle is such that melt generation should be in ‘steady state’ and U-Th characteristics should be constant. On that assumption, measured differences in collected samples can be used to calculate model ages relative to the ‘youngest’ sample, thus allowing the construction of a relative eruption timescale. However, significant fractional crystallisation is taking place in the oceanic crust, as testified by the frequent presence of plagioclase crystals up to mm-size in the glass samples. A ‘magma chamber’ on a scale larger than the magma channels is not required and we aim to assess the rate of plagioclase crystal growth using a 226Ra chronometer. This chronometer requires the assumption that Ba-Ra fractionation is constant and can then also be used to calculate a relative model age timescale, provided that not all samples are >8000 y old, which we consider unlikely.